1. Field of the Invention
This invention is directed to transducers, and more particularly to transducers for use as leak detection probes in recovery boilers. Small leaks in boiler tubes of recovery boilers eject water or steam at high pressure. Sound produced by these jets have peak frequencies ranging from about 10 KHz for a one millimeter leak to 2 KHz for five millimeter leaks. The noise produced by these jets can be detected above the background low frequency combustion noise of the recovery boiler and used to produce an alarm. Leaks in recovery boilers are dangerous, and it is therefore desirable to have a reliable and rugged device to detect such leaks. The present invention is directed to a system employing carbon granules for leak detection.
2. Description of the Prior Art
In prior art systems, conventional microphones containing a vibrating diaphragm are employed to detect leaks. Because of the harsh atmosphere of the recovery boiler, the diaphragm typically corrodes within several weeks. In addition, the positioning of the microphone is such that some of the high frequency sounds to be detected are considerably attenuated.
Microphone systems employing carbon granules are disclosed in U.S. Pat. Nos. 1,581,334 to Crandell, 2,697,136 to Baker, et al. and 3,859,478 to Cragg, et al. Generally, such microphones operate on the basis of resistance changes in the carbon granules caused by changes in mechanical pressure on the granules. However, prior art microphones employing carbon granules have all included a diaphragm. Therefore, such microphones would be faced with the same problems of corrosion in a recovery boiler atmosphere as conventional microphones.
U.S. Pat. Nos. 3,028,450 to Manning, 3,167,824 to Florer, et al., 3,209,344 to Miller, 3,831,561 to Yamamoto, et al. and 4,309,576 to Corrigan disclose detection systems for water or gas leaks. The patent to Yamamoto is specifically directed to detection of leaks in recovery boilers. None of these patents discloses or suggests the use of a carbon granule microphone.